Needleless injection devices which administer intramuscular and/or subcutaneous medications without the use of a needle are well known. Among the many advantages of such needleless injection devices are the reduction of pain and apprehension commonly associated with hypodermic needles, the elimination of needle stick injuries, and the reduction of environmental pollution associated with contaminated needle disposal. Moreover, needleless injection devices are useful in a wide range of drug therapies, including the administration of vaccines, hormone therapies and local anesthetics. Further, it is well known that such needleless injection devices are useful in the administration of insulin to the diabetic population, where individuals frequently require a number of daily injections.
Injectable medications fall into two different general categories, namely: unit dose drugs such as vaccines and analgesics; and variable dose drugs such as insulin, where the dose size must be adjusted specifically so as to meet the immediate needs of the individual at the time of administration. When a variable dose is required, as in the case of the administration of insulin, a very accurate amount of medication must be transferred to a variable dose ampule of the needleless injector. Insulin doses are typically marketed in 3 ml and 5 ml syringe cartridges, as well as being provided in bulk in a standard 10 ml medication vial.
The use of needleless injection devices has recently become of great interest, particularly by people of limited physical abilities such as the elderly, the very young and the infirm. Such persons with limited physical abilities may find the use of conventional needle syringes either difficult or impossible. Therefore, the simplified injection process associated with needleless injectors makes their use very desirable among such people.
The principles of needleless injection and the advantages of such needleless drug delivery systems over conventional hypodermic needle injection systems have long been known. However, very few needleless injection devices have achieved commercial success in the marketplace. This lack of acceptance by the user community can be attributed, at least in part, to a number of factors, chief among which are: mechanical designs which have the potential to inflict serious injury if an injector device is inadvertently fired without a medicament container or ampule in place, undesirably complex filling techniques, and the high cost of such contemporary injection devices. This cost disadvantage is particularly troublesome for those individuals who must self-administer a large number of daily injections, such as diabetics.
One existing needleless injection device is described in U.S. Pat. No. 4,874,367 to Edwards. It employs a sealed ampule that is prefilled with a selected amount of medication. The prefilled ampule is attached to a separate spring-loaded firing mechanism which, when triggered, propels a ramrod from the front of the mechanism and against a plunger located in the ampule. The ramrod drives the plunger against the medication, producing a high pressured jet for injection purposes. The plunger expels the medication from a discharge orifice and into the patient's subcutaneous tissue.
Although effective in some respects, this contemporary needleless injection device is severely limited in practical applications. In order to cock the firing mechanism, the user is required to force the ramrod back into the firing mechanism by pushing the device against a solid surface, such as a table top, until the ramrod latches behind a trigger mechanism. Thus, the strength of an individual user imposes a strict limit upon the spring force that can be utilized in the device. Many elderly, very young or infirm people simply do not have the physical strength required to cock the firing mechanism of such a contemporary needleless injection device.
Moreover, employing a spring force which is low enough to be practical for the elderly, the very young and the infirm to cock the device results in the spring force being inadequate to produce effective and reliable injection pressures for most adults. That is, such a device would generally lack the ability to penetrate the skin and subcutaneous tissue sufficiently to insure proper, reliable, operation thereof.
In addition, the firing mechanism, having a spring actuated ramrod which extends outside of the device body, has the potential to inflict serious injury if inadvertently fired without the ampule in place. For example, firing such a device without having the ampule attached thereto may result in harm to a person who is inadvertently struck with the rapidly moving ramrod.
Moreover, the fixed dose ampule of contemporary needleless injection devices such as those of the >367 patent must be prefilled at the factory and then attached to the injector when required for usage. In actual practice, however, this procedure is not practical for the simple reason that drug products cannot generally be stored in plastic containers for the extended periods of time which are typically experienced by such factory prefilled ampules. The only approved material for long term liquid medication storage is type-1 glass, which is used for virtually all drug products. However, due to the dynamics of needleless injection, in which the ampule is subjected to very high pressures during the ejection process, glass is not a suitable material for the ampule because it is too easily shattered. Consequently, it is desirable to have an ampule which may be coupled to a conventional drug vial or other medication container at the time of use, and then be filled therefrom with an accurate dose of medication. The ampule should be made of a high strength plastic material.
Indeed, the needleless injection device of U.S. Pat. No. 4,874,367 is formed of durable materials and utilizes comparatively expensive manufacturing techniques, so as to assure long time reliable use thereof. As such, this device is comparatively expensive to manufacture. The expense associated with the manufacture of this device precludes the sale and use of this device as a single use, disposable needleless injector.
U.S. Pat. No. 4,913,699 to Parsons overcomes some of the aforementioned deficiencies associated with contemporary needleless injectors. This patent describes a disposable needleless injection device having a firing mechanism that operates to release compressed gas from a storage compartment. The compressed gas acts upon a piston which drives a plunger that ejects a selected dosage of medication through an aperture in the discharge end of the device. However, the medication to be administered must first be drawn into a chamber provided in the interior of the injector before being dispensed. Thus, although being pre-cocked, and loadable (with medicine), the device is rather complicated to use. In addition, no provision is made for filling the medication chamber directly from standard medication containers. In order to fill the medication chamber, a complex liquid transfer system is required.
The device disclosed in the >699 patent is relatively complex. It is manufactured from materials able to withstand the pressures associated with a compressed gas activation system. Indeed, this needleless injection device is formed of comparatively durable materials and utilizes comparatively expensive manufacturing techniques, so as to assure long term reliable operation thereof in light of the aforementioned pressures. As such, this device is comparatively expensive to manufacture. The expense associated with the manufacture of this device precludes its sale and use as a single use, disposable item.
Thus, although the aforementioned contemporary needleless injection systems are compact and reliable, they are too complex and expensive to manufacture in order to be considered disposable.
In view of the foregoing, it is clear that there is a need in the art for a needleless hypodermic injection device which has an enhanced simplicity of design and which can be manufactured from a small number of low cost components in order to be implemented as a truly disposable system. Furthermore, there is a need for a needleless hypodermic injection system which includes a medication ampule which is capable of being filled with an accurate dose of medication and which does not impose a risk of injury due to needle use and which does not contribute to the contamination hazards attendant with needle disposal.
In this regard, it is desirable to provide a convenient, low cost and disposable needleless injector device that is configured to be conveniently and comfortably grasped in one hand of a user in a manner which facilitates self-administration of a desired medication. The system should comprise a firing mechanism and an ampule cooperating in a novel design having simplicity in both structure and function. The ampule should be such that the user may fill it with a selected dosage conveniently and accurately from existing medication vials, so as to facilitate both variable and fixed dose applications. The injector firing mechanism should be conveniently and safely operated without the need for a user to force the apparatus against a piece of furniture or the like in order to cock a spring. There should be no substantial danger associated with firing the device without an ampule in place.